Configuring EIGRP Timers

When working with EIGRP you may be required to tune the timers to meet network requirements for resiliency. This lab will discuss and demonstrate the configuration and verification of EIGRP Timers.

Real World Application & Core Knowledge

So up until now you’ve learned how to configure EIGRP neighbor relationships and how to configure which interfaces participate in the EIGRP routing process. Now its time to get further in depth with the operation of EIGRP.

EIGRP uses two separate timers to ensure neighbor relationships remain established. These timers are called the “Hello timer” and the “Hold Down Timer”. If you’re familiar with the operation of RIP then you should be able to make a very good guess as to what these timers are responsible for.

The hello timer is the interval at which a router will send “hello” messages to neighboring routers to let them know that the originating router is still online and the hold-down timer is the interval at which to consider a neighbor dead if a hello message is not received during that time window.

The default hello timer for a high-speed broadcast network link is 5 seconds and the hold-down timer is 15 seconds whereas the default timers for slow-speed NBMA link are 60 seconds hello and 180 seconds dead. A slow-speed NBMA link is classified as any NBMA link with speeds equal to or less than 1544Kbps (A single T1)

There is a common misconception that the Hello and Hold-down timers must match between routers to form an adjacency but in fact they do not need to match at all. When a router sends a hello packet to a neighboring router the hello packet includes the hold down timer which essentially tells the receiving router “If you do not hear from me in this amount of time consider me dead and get on with your router life.”

However…… There is one exception to this rule. If you have multiple routers on a network that form adjacencies then all of those routers must have matching hello/dead timers or the adjacencies will flap. This is a common problem with EIGRP in a frame-relay hub and spoke topology where a single T1 NBMA PVC does not support broadcast. In this case the broadcast PVC’s will use the hello/dead timers of 5/15 whereas the non-broadcast PVC will use 60/180. This will cause the hub to have adjacencies with neighbors with different timers on the same physical network thus causing flapping adjacencies.

If you completed the previous lab you should have noticed on all routers in the frame-relay hub-and-spoke topology that the adjacencies were flapping with the hub router. This is due to a multiple timer mismatch on the hub with one or more spokes. This lab teach you how to resolve that problem.

The next big reason as to why you may want to manually change the timers on an EIGRP routed network is to increase network outage detection and re-convergence time. By default on high speed links the hello/dead timer is 5/15 so with that in mind if a router goes down it will take up to 15 seconds before the neighboring routers know about this outage and then begin to reconverge on the outage. In some networks its idea to have the ability to detect router outages as quick as possible and in this case you can configure the hello timer to 1 second and dead timer to 3 seconds.

The EIGRP Hello and Hold-Down timers are configured on a per-interface basis using the ip hello-interval eigrp AS# timeinseconds# and ip hold-time eigrp as# timeinseconds# commands in interface configuration mode.

Familiarize yourself with the following new command(s);

Command

Description

ip hello-interval eigrp AS# timeinseconds#

This command is executed in interface configuration mode to manually configure an EIGRP hello interval on a per-interface per autonomous system basis. Time is specified in seconds.

ip hold-time eigrp as# timeinseconds#

This command is executed in interface configuration mode to manually configure an EIGRP dead interval on a per-interface per autonomous system basis. Time is specified in seconds.

This lab will continue to build upon the topology previously used in Lab 8-4 and other labs found through out Section 8.

Lab Prerequisites

If you are using GNS3 than load the Free CCNA Workbook GNS3 topology than start devices; R1, R2, R3, R4 and R5.

Establish a console session with devices R1, R2, R3, R4 and R5 than load the initial configurations provided below by copying the config from the textbox and pasting it into the respected routers console.

Lab Objectives

Configure EIGRP on R4 to send Hello’s to R1 at 5 seconds and a dead timer of 15 seconds.

Verify your configuration on R1 by using the show ip eigrp neighbor command.

Lab Instruction

Objective 1. – Configure EIGRP on R4 to send Hello’s to R1 at 5 seconds and a dead timer of 15 seconds.

To complete this objective you’ll use the two commands discussed in the core knowledge section of this lab as shown below;

Objective 2. – Verify your configuration on R1 by using the show ip eigrp neighbor command.

You can easily determine the hello/dead timers of an EIGRP neighbor by viewing the neighbor adjacencies. If the hold timer is less then 15 seconds then its safe to assume that the neighbor is using a 5 second hello interval and a 15 second dead timer. You can view the neighbor table multiple times to see that the hold timer is reset back to 15 seconds upon each receipt of a hello packet as shown below;